Fluorinated ionomers are used commercially in the manufacture of ion exchange membranes for electrochemical applications. One industrial application is the chloralkali process for the electrolysis of sodium chloride to produce sodium hydroxide and chlorine. Polymer electrolyte membrane (PEM) fuel cells typically employ fluorinated ionomer membranes. Fluorinated ionomer is also useful as a solid acid catalyst for various chemical processes.
Most known processes for the polymerization of fluorinated ionomer copolymerize tetrafluoroethylene with a fluorinated vinyl ether monomer having sulfonyl fluoride group such as is disclosed in U.S. Pat. No. 3,282,875, U.S. Pat. No. 4,940,525 and U.S. Pat. No. 7,071,271. The process of U.S. Pat. No. 3,282,875 teaches that it is preferred to carry out polymerization in a perfluorocarbon solvent. U.S. Pat. No. 4,940,525 teaches an aqueous polymerization process using the ammonium salt of perfluorooctanoic acid as a fluorosurfactant. U.S. Pat. No. 7,071,271 teaches a process which employs the step of making a pre-emulsion of the SO2F-comonomer using a microfluidizer prior to the start of the polymerization process. The pre-emulsion, however, is not stable over time, i.e., has a “pot life” of at least 1 hour.
Moreover, the copolymerization of tetrafluoroethylene with a fluorinated vinyl ether monomer with a sulfonyl fluoride group does not produce an ionomer directly and instead produce a fluorinated ionomer precursor. Although the sulfonyl fluoride precursor is useful for fabrication of articles such as membranes by melt processing, the fluorinated ionomer precursor must be subjected to a subsequent process step to hydrolyze the sulfonyl fluoride groups of the polymer sulfonate groups to form the ionomer. When ionomer is desired is sulfonic acid form, the ionomer must be further treated to convert it to acid form.
A process is desired for the manufacture of fluorinated ionomer which directly produces fluorinated ionomer, especially as a dispersion of fluorinated ionomer particles in an aqueous medium.